The invention described herein relates generally to wind turbines. More specifically, the invention relates to a method and system for configuring the default states of brakes in a wind turbine.
Recently, wind turbines have received increased attention as an environmentally safe and relatively inexpensive alternative energy source. With this growing interest, considerable efforts have been made to develop wind turbines that are reliable and efficient.
Generally, a wind turbine includes a rotor having multiple blades. The rotor is mounted within a housing or nacelle, which is positioned on top of a truss or tubular tower. Utility grade wind turbines (i.e., wind turbines designed to provide electrical power to a utility grid) can have large rotors (e.g., 80 or more meters in diameter). Blades on these rotors transform wind energy into a rotational torque or force that drives one or more generators, rotationally coupled to the rotor through a gearbox. The gearbox may be used to step up the inherently low rotational speed of the turbine rotor for the generator to efficiently convert mechanical energy to electrical energy, which is fed into a utility grid.
The design of the blades and the tower of a wind turbine are often dimensioned by the extreme loads that occur during storm winds combined with grid loss. Even though the turbine blades are fixed at an angle close to ninety (90) degrees, they are not really in a feathered position because the lack of grid power prevents the wind turbine from yawing towards or away from the wind direction. Extreme loads in the blades and tower are produced by the force of strong storm winds that hit a large surface area of the blade (lateral yaw direction) and nacelle. The storm loads may be alleviated by providing a source of secondary power, such as a diesel generator, in order to yaw the turbine towards or away from the incoming wind. For example, one such method keeps the plane of rotation of the rotor substantially perpendicular to the direction of wind. The blade angle of the rotor blades are adjusted to a minimum operating angle close to ninety (90) degrees for spinning the rotor and the generator to produce the necessary power to turn the rotor and to keep the rotor toward the incoming wind during storm loads. However, it is desirable to alleviate the need for a separate backup generator because of the extra cost and complexity associate therewith.
In addition, the yawing of the wind turbine to keep the rotor perpendicular to the wind direction may be prevented due to the lack of grid or back-up power. In some applications, it may be desirable to have the wind turbine go downwind in storm and grid loss. The lack of power can potentially prevent this. In addition, yawing of the turbine can only be done very slowly (about 0.5 degrees/s) due to load constraints on the turbine structure. Hence, if the wind direction changes rapidly, one will not be able to keep the rotor perpendicular to the wind direction and the strategy used in conventional wind turbines will not have the desired outcome. Many systems use a motor brake that has a brake torque demand during grid loss, and these brakes are typically provided with a normally-closed function. That is, when power loss occurs the brakes close and prevent movement of the relevant structures. These brakes exert the same brake torque during application in operation (e.g., at yaw runaway during a grid loss state). For turbines designed to go downwind during storms or grid loss events, this normally closed brake configuration may result in a brake torque that is too high.